SPC Apr 7, 2017 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0750 AM CDT Fri Apr 07 2017

Valid 071300Z - 081200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE INTERIOR NORTHWEST AND NORTHERN ROCKIES...

...SUMMARY...
A few thunderstorms associated with hail and strong wind gusts will
be possible across eastern Oregon, eastern Washington, northern
Idaho, and western Montana today.

...Synopsis...
A high-amplitude upper-air pattern prevails over the contiguous U.S.
and surroundings this morning, featuring well-defined cyclones
centered over NY and offshore from OR.  A positively tilted,
synoptic-scale trough extending south-southwestward from the cyclone
over the Northeast will move offshore from most of the Atlantic
Coast through the period, and already has been preceded by a
seasonally strong cold front now crossing the Bahamas.  An extensive
continental/polar air mass behind that front will render thunder
potential negligible east of the Rockies through the period, thanks
to feeble moisture and weak to no convective instability. Farther
west, synoptic-scale ridging now from the southern Rockies to
western Hudson Bay will move slowly eastward, reaching the southern
High Plains, mid Missouri Valley and southern Hudson Bay by the end
of the period.  

The Pacific cyclone is in the process of devolving into a open-wave
trough, which will eject north-northeastward toward Vancouver Island
while weakening markedly.  This will occur as a mid/upper-level
speed max and vorticity banner -- presently evident in
moisture-channel imagery over the Pacific high seas well south of
the Gulf of Alaska -- amplifies in prevailing westerlies and moves
rapidly eastward.  The latter perturbation will approach the CA
coast at the end of the period and proceed inland early day-2.  A
vorticity max and related shortwave trough that already is inland
over southern WA and eastern OR will eject east-northeastward across
northern ID to northwestern MT through 00Z.

...Interior Northwest...Northern Rockies...
Several factors will juxtapose to support thunderstorm potential
today across the outlook area, with isolated severe hail and/or
gusts possible:
*  Low-level warm advection early, followed by
*  Steepening of low/middle-level lapse rates related to
*  Diabatic surface heating behind an ongoing arc of thick mid/high
cloud cover over the region, acting to weaken already minor MLCINH
and
*  DCVA-related cooling aloft, collocated with
*  Thermally indirect ageostrophic circulation within the exit
region of an elongated, cyclonically curved, 125-140-kt upper jet
streak. 

Time series of forecast soundings suggest that related
destabilization, atop weak but sufficient low-level moisture, will
support MLCAPE up to about 250 J/kg.  Except where locally modulated
in the boundary layer by orographic forcing, vertical wind profiles
generally should be unidirectional with around 35-45 kt
effective-shear magnitudes -- mostly arising from speed shear.  Lack
of more robust low-level theta-e (especially on the southern end
where drier air may advect northward from parts of the northern
Great Basin), concerns over sufficient deep-convective coverage, and
the somewhat early timing of strongest large-scale lift, combine to
preclude a better-organized severe threat.

..Edwards.. 04/07/2017

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